Sunglasses on Mercury

I was having an email chat with Emily at the Planetary Society blog about the spectral capabilities of the MESSENGER spacecraft yesterday. It turns out that she has an excellent summary of the camera system on that website. We were talking about filters and false color schemes. Evidently, one of her readers was wondering what Mercury would look like to an astronaut in orbit. It was basically a true color vs spectral recombination question. What we were wondering was whether or not that astronaut would be blinded by the glare from the much higher illumination. So here are some back-of-the-spreadsheet calculations, using 1500 w/m² for earth insolation, and various orbital and albedo values from the web (mostly wikipedia).

Planet	w/m2	albedo	distance	reflected w/m2	notes
Earth	1500	0.8	150,000,000	1200	snow
Earth	1500	0.35	150,000,000	525	desert
Venus	2894	0.65	108,000,000	1881
Mercury	6927	0.15	69,800,000	1039	aphelion
Mercury	15950	0.15	46,000,000	2392	perihelion

Venus and Earth both have orbits that are almost circular, but Mercury is the most eccentric planet that hasn’t been cruelly demoted, so the insolation at perihelion is more than twice what it is at aphelion. As the table above shows, the fairly dark mercurial surface is slightly less bright than fresh Earthly snow when Mercury is farthest from the sun, but it is almost twice as bright when the planet is at its closest approach. So an astronaut could very easily go snowblind from looking at it. On the other hand, mountaineering goggles would be more than adequate when near apahelion. But the original question was about ‘true’ color. Sunglasses are rarely spectrally neutral, so as soon as they go on, true color disappears.

In fact, when we log drill cuttings in the desert, the time when sunnies go on in the morning and come off in the afternoon is noted, because it can change the reported rock color.